The invention relates to vacuum operated valves, and more particularly to a combination vacuum delay and vacuum shutoff apparatus which may be employed in an automotive exhaust gas recirculation system.
Exhaust gas recirculation (EGR) is widely used to control the emission of oxides of nitrogen (NO.sub.x) from internal combustion automotive engines. In such an EGR system, an electronic servo system often is employed to operate the EGR valve, which in turn opens a line from the intake manifold to a source of exhaust gas from the engine. The electronic systems, however, are generally quite costly as either original or added-on equipment.
Another means which has been used to operate the EGR valve has been a vacuum line connected to a ported vacuum source at the carburetor, such as that often used for the vacuum spark advance. When vacuum from the ported source reaches a predetermined level, a diaphragm opens the EGR valve. However, this type EGR actuating system results in EGR during certain engine and vehicle performance conditions wherein EGR is not desirable, and in lack of EGR during certain conditions wherein the recirculation is desirable. For example, when the automobile is in a condition of moderate acceleration from a stop or from a low speed, climbing toward a cruise speed, the ported vacuum source in the carburetor would provide sufficient vacuum to open the EGR valve and permit recirculation. However, exhaust gas recirculation should not be immediately initiated by this condition since not enough air is being drawn through the carburetor and manifold and into the cylinders during this period to maintain smooth engine operation. Recirculation should not commence until approximately when the cruise speed is reached, when air flow into the cylinders is adequate and when the engine is under less demanding loading conditions. Similarly, when the vehicle is at a sustained low speed cruise with the throttle opened only a small amount or when the vehicle is decelerating from a high speed and the throttle is only slightly open, exhaust gas recirculation takes place. Again, EGR is undesirable under these conditions because there is very little air flow through the carburetor and a relatively high vacuum exists in the intake manifold, causing a large EGR flow. The relatively inert exhaust gas thus tends to fill the firing cylinders and when an acceleration is called for there may occur a significant hesitation or engine roughness.